The invention relates to a device for the combined measurement of the arterial oxygen saturation and the transcutaneous carbon dioxide partial pressure on the ear.
It is known to measure the oxygen saturation of the haemo-globin in arterial blood (arterial oxygen saturation) by means of a noninvasive optical method which is referred to as pulse oximetry. The principle of this method is based on measuring and evaluating changes in the absorption of light caused by the pulsatile inflow of arterial blood into a well-perfused part of the body (e.g. finger pad or ear lobe). The SpO2 measured in this way normally provides reliable information about the patient""s oxygenation. Pulse oximetry is routinely employed in various medical fields, in particular for intra- and postoperative patient monitoring.
However, information about oxygenation is not always sufficient on its own. It is frequently necessary also to know the arterial carbon dioxide partial pressure (paCO2) in order to be able to assess the patient""s respiratory functions. The methods currently available for measuring the paCO2 are essentially the three described below:
1. Removal and analysis of an arterial blood sample. Although this method allows direct measurement of the paCO2, it has the disadvantage that it is invasive and requires access to an artery. In addition, the measurement is not continuous and therefore does not allow changes in the paCO2 to be monitored continuously. The method has the further disadvantage that the analytical result is usually available only after a delay of several minutes.
2. Capnometry. This is an optical absorption measurement in the infrared region used to determine the concentration of CO2 in the expired gas mixture. The paCO2 can be calculated from the CO2 concentration in the end-expiratory phase. However, as indirect method, capnometry has the disadvantage that it does not always correctly reflect the paCO2. Thus, it is known that this value is often an underestimate to varying extents. It is also possible for other parameters, e.g. a change in the cardiac output, to result in a change in the end-expiratory CO2 concentration and thus cause an incorrect estimate of the paCO2.
Furthermore, the possible applications of capnometry are restricted by the fact that it can be employed in general only for intubative, artificially ventilated patients. It is therefore in general impossible to determine the paCO2 by capnometry during operations on nonventilated patients. Nor is capnometry suitable for monitoring the transition phase from artificial ventilation to spontaneous breathing. It is precisely during such a transition that continuous measurement of the paCO2 is often required.
3. Transcutaneous PCO2 measurement. This method is likewise indirect and makes use of the fact that carbon dioxide is able easily to diffuse through body tissue and skin. The gas is measured with a sensor attached to the surface of the skin. When a sensor of this type is warmed to a temperature of about 41xc2x0 C. to about 45xc2x0 C., this produces local dilatation and arterialization of the capillary bed at the measurement site. Under these conditions, the transcutaneous carbon dioxide partial pressure (tcpCO2) measured there shows a good correlation with the arterial value. This makes it possible, with certain restrictions, to determine the paCO2 with an accuracy which is sufficient for most applications.
Detailed information about the measurement methods mentioned and their clinical applications may be found, for example, in the review article xe2x80x9cNoninvasive Assessment of Blood Gases, State of the Artxe2x80x9d by J. S. Clark et al., Am. Rev. Resp. Dis., Vol. 145, 1992, pp. 220-232.
Of the abovementioned methods for paCO2 measurement, at first sight the transcutaneous method appears to have the most advantages: this measurement is noninvasive, continuous and can also be employed for nonintubated patients. Nevertheless, transcutaneous PCO2 measurement has not to date become widely used for intra- and postoperative patient monitoring. It is employed for this only extremely rarely, whereas it has long been established as a routine method in other medical fields, for example in intensive monitoring in neonatology.
One of the reasons for this is that the sensors currently available for tcpCO2 measurement are suitable for application only to sites on the body to which access by the anaesthetist during an operation is usually difficult: a tcpCO2 sensor must be applied by means of an adhesive ring which is adhesive on both sides to a well-perfused, hairless site of low convexity on the skin, with the diameter of the area of skin covered by the sensor and adhesive ring being about two to three centimetres. Particularly suitable measurement sites are therefore the thorax region, the abdominal regions and the inside of the upper arm or thigh. These sites are, however, not directly accessible for the anaesthetist, and can usually not be inspected either if they are covered. Thus, for example, it is difficult to check whether the sensor is adhering well or has become detached. Possible repositioning of the sensor during the operation is also difficult. In addition, on these sites on the body, the sensor may impede the surgeon or conflicts with the requirements for sterility in the vicinity of the operative field may occur. In addition to these difficulties which derive from the measurement site, application of a tcpCO2 sensor is often regarded as complicated because a contact gel must be applied in order to avoid inclusions of air between sensor and skin. The dosage of this gel is critical because if the amount is too large the adhesion area of the adhesive ring would be wetted and, in this case, satisfactory attachment of the sensor would no longer be ensured. On the other hand, too small an amount of the gel would be ineffective.
There are no difficulties of this nature on application of a pulse oximeter sensor. This is usually attached by means of a clamp or an adhesive strip to a finger or an ear lobe. No special complexity is required for this. In contrast to a transcutaneous sensor, no contact gel is required. The measurement site on the ear is in particular usually easily accessible and easy to inspect by the anaesthetist. It is extremely rare for the surgeon to be impeded or problems to arise with the requirements for sterility there.
However, the disadvantage of pulse oximetric measurement on the ear lobe is that the signal measured there is often very weak. On the one hand, this derives from the fact that the thickness of the tissue from which the signal is obtained is relatively small by comparison with the finger pad. On the other hand, the ear lobe is often cold, as a result of the frequently low temperature in the operating theatre, and therefore poorly perfused. This may result in the signal measured on the ear being so weak that pulse oximetric measurement is no longer possible there. The anaesthetist is then forced to carry out the measurement on a finger. Although the finger is in principle also easily accessible as measurement site, its location is less favourable from the anaesthetist""s point of view, who normally does his work near the patients head. An additional factor is that an arterial catheter or a cuff for measuring blood pressure is frequently attached to the patient""s arm. Such an arm must not be used for pulse oximetric measurements because the latter would be impaired otherwise. The other arm is frequently less accessible, depending on the patient""s position.
It may be stated in summary that the problems of monitoring the arterial oxygen saturation and the arterial PCO2 in patients during and after surgical operations have by no means been satisfactorily solved yet.
The object of the present invention is therefore to provide a device which makes simple and reliable measurement of these two parameters possible on the measurement site preferred by the anaesthetist, the ear. It is additionally intended to ensure that a signal strength sufficiently high for pulse oximetric measurement is available.
This object is achieved according to the invention by a device for the combined measurement of the arterial oxygen saturation and the transcutaneous CO2 partial pressure on an ear lobe, having a sensor which has means for pulse oximetric measurement of the arterial oxygen saturation, means for measuring the transcutaneous CO2 partial pressure and means for warming a sensor contact surface intended for contact on the ear lobe.
For better understanding of the invention, firstly the two measurement processes will be described in somewhat more detail:
Pulse oximetric measurement makes use of the fact that the absorptivities of haemoglobin for light differ in its oxygen-saturated and its reduced forms. The absorption coefficient of blood for red light depends greatly on the oxygen content and is virtually independent thereof for light in the near infrared region. It is possible by measuring the ratio of the intensities of the light absorbed at the two wavelengths to determine the arterial oxygen saturation. The light sources normally used are two diodes (LED) which are located close together and have wavelengths of about 660 nm (red) and 890 nm (infrared). The light emitted by the LEDs is passed into a well-perfused part of the body and is there scattered and partly absorbed. The light emerging again from the part of the body is measured by a photodiode which is usually disposed opposite to the LEDs. The light measured by this photodiode at the two wavelengths consists of a stationary and a time-dependent component. The stationary component is essentially determined by absorption by bones, tissues, skin and non-pulsatile blood. The time-dependent component is caused by changes in absorption in the object of measurement elicited by the pulsatile flow of arterial blood. To determine the arterial oxygen saturation, the quotients of the pulse-modulated and the stationary components are formed separately for the two wavelengths. These quotients represent the primary signals measured. The SpO2 is calculated from their amplitudes by means of an empirically determined function. The sensitivity of the pulse oximetric measurement is limited by the fact that interfering signals and electronic noise are superimposed on the signals measured. If the primary signals measured are too weak, reliable determination of the SpO2 is no longer possible. The primary signals measured on the ear lobe are usually weaker by a factor of about 10 than the values measured on the finger, which is attributable mainly to the smaller thickness of tissue on the ear. It is easily possible with this signal strength which is low in any case, as mentioned above, for the pulse oximetric measurement on the ear lobe no longer to be possible if it is very poorly perfused.
Transcutaneous pCO2 measurement is based on an electrochemical principle. The measurement takes place potentiometri-cally by determining the pH of a thin layer of an electrolyte solution which is coupled to the skin via a hydrophobic membrane which is very gas-permeable. A change in the pCO2 on the surface of the skin causes a change in pH of the electrolyte solution, which is proportional to the logarithm of the pCO2 change. The pH is determined, for example, by measuring the potential between a miniature glass pH electrode and an Ag/AgCl reference electrode. The tcpCO2 sensor contains a heating element which heats it to a temperature of about 41xc2x0 C. to 45xc2x0 C. As mentioned at the outset, this produces local dilatation and arterialization of the capillary bed at the measurement site, which causes the tcpCO2 measured there to correlate well with the arterial value.
The essential feature of the invention described hereinafter is that the two measurement functions for SpO2 and tcpCO2 are integrated into one unit so that simultaneous measurement thereof on the ear lobe is possible with an accuracy and reliability which is sufficient for clinical requirements. An important component of this unit is, besides the actual measuring part, the appliance for attaching the sensor to the ear.
The fact that such a combined sensor has not previously been disclosed, despite the evident clinical demand, is attributable to various reasons:
On the one hand, the necessary miniaturization, especially of the tcpCO2 measuring part, involves considerable design difficulties which, however, will not be dealt with in detail because the invention does not relate to the solutions used therefor. On the other hand, it was not to be expected directly that the two very different measurement functions can be combined in a miniaturized unit in such a way that they do not have adverse effects on one another and that no other disadvantageous effects occur either. Thus, for example, there were doubts about whether the potential of the Ag/AgCl reference electrode could be influenced by the light used for the SpO2 measurement, as a result of a photochemical reaction. However, no such influence has been detectable.
On the other hand, another concern proved to be justified: it was suspected that some of the components required for the tcpCO2 measurement would cause an optical shunt which might impair the SpO2 measurement. This proved to be correct and resulted, in a preferred embodiment, in specific design measures which will be dealt with in detail hereinafter.
Apart from these purely technical doubts, it was unknown whether the ear lobe would in fact be suitable, for physiological reasons, for transcutaneous measurement of the pCO21 that is to say whether measurement there is possible with adequate accuracy and a sufficiently short reaction time (in vivo response time) to changes in the arterial pCO2. Thus, for example, transcutaneous measurement on the finger pad cannot, although it is also well perfused, be used for clinical application because the in vivo response time there is too long. Nor was it directly evident whether a reliable and sufficiently stable attachment of the sensor to the ear lobe is possible without impeding perfusion in the capillary tissue near the surface on the measurement site. Even slight application of pressure (for example by the force of the spring of an earclip) may cause such an impediment. Although this would be relatively uncritical for pulse oximetric measurement, emptying of the blood capillaries near the surface may cause considerable problems for the transcutaneous measurement: these comprise, on the one hand, the fact that the carbon dioxide gas diffuses to the surface of the skin from relatively deep-lying capillaries and thus the diffusion pathway is extended. This may lead to falsification of the measured result and a prolongation of the in vivo response time. On the other hand, in the extreme case, the absence of perfusion near the surface may also lead to burn injuries if the heat supplied by the sensor is not removed quickly enough. This is a danger especially with high sensor temperatures of 44xc2x0 C. and 45xc2x0 C., which are close to the critical value for protein decomposition to start. The provision of a suitable appliance for attaching the sensor to the ear is therefore an important and integral component of the invention.